Nonlinear Ultrasonic Measurements Research Articles

Sideband Peak Count – a new nonlinear ultrasonic technique for monitoring damage progression in engineering materials

Linear ultrasonic (LU) techniques used by majority of the researchers working in material damage monitoring, are reliable for detecting relatively large defects. If the defect dimensions are in the range of the ultrasonic wavelength or larger, then those defects can be detected by analyzing the scattered ultrasonic fields. Material damage affects LU parameters such as ultrasonic wave speed and attenuation and their changes are detectable for relatively large defects. However, for small defects (when defect dimensions are significantly smaller than the wavelength of the propagating signal) the changes in the LU parameters are too small to detect or measure reliably. For detecting small defects engineers often use high frequency ultrasonic signals to make the defect dimensions greater than the wavelength. However, high frequency signals attenuate quickly and therefore, only very small regions near the ultrasonic probe can be inspected in this manner. Inspecting large structures by high frequency ultrasonic signals requires moving the probe mechanically from one point to the next and therefore, can be very time consuming. Nonlinear ultrasonic (NLU) technique on the other hand does not need to satisfy this restricting condition that the wavelength of the signal must be smaller than the defect size. NLU works well when the signal wavelength is much larger than the defect size. Therefore, relatively low frequency signals that can propagate a long distance and monitor a large area of a structure can be used for NLU measurements. Different NLU techniques can be used for detection and monitoring of small damages in a specimen. A relatively new NLU technique called the Sideband Peak Count-Index or SPC-I technique has been developed by the author and his collaborators. SPC-I technique for monitoring different types of materials – composites, metals, concrete, and other cement-based materials has been found to be effective. Along with those success stories of SPC-I the effect of topography and the advantage of topological sensing is also discussed in this presentation.

Comparative study of linear and nonlinear ultrasound applied to the detection of hydrogen damage in 7N01 aluminum alloy

7N01 aluminum alloy samples with different hydrogen damage degrees were prepared by electrochemical hydrogen charging technology. 7N01 aluminum alloy samples with different degrees of hydrogen damage were characterized by metallographic observation, hardness test and XRD test. The results show that the hydrogen content increases with the increase of hydrogen charging time. The surface of aluminum alloy is exfoliated and pits appear. The more severe the hydrogen damage, the greater the depth of pits. The microhardness of the 7N01 aluminum alloy decreases after hydrogen damage, which only occurs near the surface. After electrochemical hydrogen charging, AlH3 appears in the structure of 7N01 aluminum alloy, which is the result of increased hydrogen concentration. The ultrasonic echo signals of hydrogen damaged samples were obtained by a high frequency longitudinal probe ultrasonic detection device, and the results of linear and nonlinear ultrasonic detection were compared. Traditional linear ultrasonic detection parameters such as sound velocity and attenuation coefficient do not change significantly in the early stage of hydrogen damage, but increase significantly in the late stage of hydrogen damage. Due to the change of microstructure, the nonlinear coefficient increases approximately linearly in the early stage of hydrogen damage and decreases in the late stage of hydrogen damage. This study demonstrates the potential for combining linear and nonlinear ultrasonic measurements in hydrogen environment to more comprehensively study hydrogen damage.

Creep strain prediction in power plant material via ANN modelling of nonlinear ultrasonic test results

ABSTRACT Reliable and accurate prediction of the creep life of power plant components is crucial for both economic and safety reasons. Existing prediction models, based on creep test data, can be complex and time-consuming. Nonlinear ultrasonic (NLU) is a widely-accepted non-destructive testing (NDT) technique for evaluating damage progression in crept specimens. The information from NLU measurements alone is insufficient to forecast the life of any component. In real-time applications, intelligent NDT protocols are needed to enable fast and accurate life prediction of such components. A methodology for creep life prediction using artificial neural networks (ANN) has been introduced based on NLU test results of crept P92 steel specimens. The technique involved creep tests of P92 specimens exposed to a temperature of 625⁰C with applied stress ranging from 120MPa to 160MPa, NLU measurements at each step load, and prediction of creep life of the material with a ANN trained with creep strain and NLU test data. The technique involves prediction from previously generated historical data, thus saving both cost and time of conducting continuous experiments. This approach for ANN modeling of NLU data can be considered a reliable, time-saving, and effective technique for assessing creep damage progression in power plant components.

Frequency sweep study on the generation of dual-mode second harmonics (DMSH) on an isotropic nonlinear elastic cylindrical rod by T(0,1) mode

Nonlinear ultrasonic (NLU) guided wave (GW) measurements are more sensitive to microscale defects than linear measurements. This nonlinear phenomenon is helpful for early-stage damage detection in material for nondestructive evaluation (NDE) and structural health monitoring (SHM) applications. The amplitude of the higher harmonics generated in the material serves as the basis for the NLU measurement. Recent studies have reported the presence of dual-mode second harmonic (DMSH) on an isotropic nonlinear elastic plate and cylindrical structures. The fundamental non-dispersive mode shear horizontal SH0 and torsional T(0,1) modes can generate simultaneously propagating dual-mode second harmonic(DMSH) on plate and cylindrical guided media respectively. For the case of cylinders, at phase matching condition two dominant second harmonic fundamental longitudinal (l(0,1)) and orthogonal torsional (t(0,1)┴) mode are generated with significant amplitude. The particle vibration of the t(0,1)┴ mode was present in both orthogonal directions compared to conventional T(0,1) mode and hence called orthogonal torsional mode. The present work aims to study the behaviour of DMSH at different frequencies through a frequency sweep study on a weakly nonlinear elastic cylinder of circular cross-section via numerical simulations and validated experimentally for some selected cases. The wave propagation of the T(0,1) mode at several selected frequencies from 300 kHz to 2.2 MHz is chosen to explore the second harmonic mode for better understanding. At frequencies below 1.25 MHz, l(0,1) mode is faster than t(0,1)┴ mode; above 1.25 MHz, it was the other way around. The behaviour of harmonics was observed to be in accordance with the group and phase velocities at the corresponding frequencies in the dispersion curves. The evidence for the presence of DMSH was validated experimentally for some selected frequency cases at different propagation distances. The significance of DMSH is explored via numerical simulations and found that t(0,1)┴ mode is sensitive towards material degradation, while all other generated wave modes were not affected. Research shows that enough energy is available in the generated DMSH to be noticed experimentally and in numerical simulations. This improved understanding of the generation of DMSH across different frequencies would be helpful in new advanced NLU-based NDE and SHM applications.

Nonlinear ultrasonic techniques to quantify oxidation damage in carbon/carbon composite material

This research applies linear and nonlinear ultrasonic (NLU) techniques to quantify oxidation damage in a carbon/carbon (C/C) composite material cut from an automotive friction disc ring. The oxidation as a result of gasification of carbon at high temperatures (T >∼ 500 °C) is a major degradation mechanism in C/C composites. This is the first research to successfully use a dry-contact mediator wedge setup for nonlinear ultrasonic applications, enabling repeatable in-situ Rayleigh wave measurements on slow wave velocity materials where submersion is impractical. The samples are subjected to thermal damage by exposure to a high temperature atmosphere over multiple holding times, and the oxidation damage is visualized using scanning electron microscopy (SEM). The sample mass is recorded and both linear (velocity and attenuation) and nonlinear (β′) ultrasonic measurements are performed after each holding time. Superior sensitivity to early stage oxidation affirms the suitability of nonlinear Rayleigh wave measurements for the quantification of thermally induced oxidation damage in C/C composites, while the double mediator wedge setup is well suited for high attenuation, slow wave velocity materials, demonstrating potential applicability for in-situ condition monitoring. The results show that a systematic oxidation starts after 3 min of exposure to the temperature in this C/C composite. The oxidation induced fiber–matrix debonding, matrix defects (appearing to be microcracking), and void growth are consistently detected by the Rayleigh wave velocity and acoustic nonlinearity. In addition to the higher sensitivity, the nonlinearity parameter has the capability of selectively measuring the strength related damage such as the observed fiber–matrix debonding, while the wave velocity can detect the oxidation voids. A combined use of these two parameters can provide useful information on the microstructural evolution in C/C composites subjected to high temperatures.

Using nonlinear ultrasonic measurements to estimate parameters of the sedimentation of slurry solid phase in thickener

Purpose. Improvement of the method for estimating sedimentation process parameters of the slurry solid phase particles in thickener on the basis of measurements of the nonlinear characteristics of ultrasonic waves propagating in a controlled medium. Methodology. The following methods were used: analysis of scientific results and practical developments; methods of mathematical statistics for evaluating the results of experiments; methods of analytical synthesis; methods of numerical modeling for the synthesis and analysis of a mathematical model. Findings. It was established that the nonlinearity of the process of propagation of ultrasonic waves in ore slurry, which is determined by the number and size of crushed ore particles in it, can be estimated by determining the amplitudes of several harmonics of the measured acoustic signal. This approach allows maintaining the productivity of the desliming process in accordance with the characteristics of the ore suspension, without allowing the loss of valuable components. Obtaining real-time information about the characteristics of the particle sedimentation process in the ore suspension at the initial stage enables reducing the duration of transitional processes in the control system. Originality. The method for estimating the parameters of the deposition of the solid phase of the pulp in the thickener has been improved, which is based on the fact that the nonlinearity of the process of propagation of a controlled ultrasonic signal in the thickener; this leads to modulation of the generated ultrasonic packet and, consequently, the appearance of higher harmonics. To obtain a more accurate estimate of the nonlinearity of this process, all the values obtained must be normalized to represent only the relative changes in the acoustic nonlinear response. Practical value. A system for automatic control of the thickener operation is proposed, which uses nonlinear ultrasonic measurements to estimate the parameters of the sedimentation of the solid phase of the pulp in the thickener. According to the results of industrial tests of the system of automatic control of the thickener based on ultrasonic control devices, it was established that its use as part of the automated control system for the technological processes of beneficiation of iron ore raw materials at the ore beneficiation factory of the Northern mining works will allow reducing water consumption by 3.5 % and iron-magnetite losses by 0.6–0.7 %.

Non-destructive evaluation of the effects of exposure environment in mortars using non-linear ultrasonic measurements

The development of non-destructive techniques has currently a high interest, such as those based on non-linear ultrasonic measurements. In this work it has been assessed the possible application of non-linear ultrasonic measurements for getting information related to the microstructure and properties of Portland cement mortars exposed to real and non-optimum environments. The pore structure was studied using mercury intrusion porosimetry and differential thermal analyses. Mechanical strengths and linear ultrasonic pulse velocity were determined. Regarding non-linear ultrasonic measurements, the results of parameters R and DIFA were analyzed. The tests were performed at 28 and 250 days. At later ages, a loss of microstructure refinement and a worsening of mechanical performance of the mortars were observed. The results of non-linear ultrasonic parameters R and DIFA overall agreed with those obtained for the microstructural and mechanical characterization performed. In particular, an adequate correlation between parameter DIFA and the pore size distributions was observed.

In situ nonlinear Rayleigh wave technique to characterize the tensile plastic deformation of stainless steel 316L

The acoustic nonlinearity parameter β is sensitive to dislocation parameters, which continuously change during plastic deformation. Dislocation-based damage in structures/components is the source of the failure; thus, β has been studied as a metric for non-destructive evaluation. This work consists of two parts: the development of an in situ experimental setup for nonlinear Rayleigh wave measurements, and characterization of the dependence of β on applied stress at different levels of initial plastic strain. First, we introduce an experimental setup and methods for repeatable in situ nonlinear ultrasonic measurements. Details on design considerations and measurement schemes are provided. In the second part, β was measured in situ during an incremental monotonic tensile test. The measured β monotonically decreases with plastic strain, but it is relatively insensitive to the applied stress during elastic deformation. This result highlights three aspects of the evolution of β, which have not been sufficiently emphasized in prior work: the apparent insensitivity of β to the applied stress during elastic deformation, decreasing β with plastic deformation, and the saturation of β. We attribute the trend of decreasing β to a scaling of β with monopole loop length during plastic deformation, which depends on initial microstructure. The saturation of β at 1.8% coincides with a planar-to-wavy transition of dislocation structures. The in situ nonlinear ultrasonic experimental method presented in this work is significant as the in situ results can provide broader insights on β and dislocation-based damage evolution than ex situ measurements alone.

Control of low-frequency guided elastic wave modes in a hollow pipe using a meta-surface

A locally resonant meta-surface for preferential excitation of a guided mode in a hollow pipe can improve ultrasonic guided wave inspection of pipelines. The proposed meta-surface comprises a periodic arrangement of bonded prismatic rod-like resonators in the circumferential and axial directions of the pipe. We demonstrate the presence of bandgaps for the low-frequency axisymmetric longitudinal modes L(0,1) and L(0,2) and the torsional mode T(0,1). The generated bandgaps can be used to filter the higher harmonics associated with the system nonlinearity to improve nonlinear ultrasonic measurements on pipes. These bandgaps exist even for the non-axisymmetric flexural modes but with their hybridized dispersion curves exhibiting mode-coupling for higher circumferential orders. Moreover, a “partial” bandgap is obtained where preferential transmission of the L(0,2) mode over L(0,1) is possible. We discuss the potential advantages of this partial bandgap to improve pipeline inspections using the L(0,2) mode. Time-domain finite element analyses are used to validate the presence of these bandgaps under radial, circumferential, and axial excitation that mimics the excitation using a ring of piezoelectric transducers. Finally, we discuss the influence of resonator spacing, filling fraction, and the number of resonator rings on the bandgaps for an informed meta-surface design.

Investigation of creep degradation in a Super304H austenitic steel and its nonlinear ultrasonic assessment method

ABSTRACT This paper reports the degradation behaviour of Super304H during the creep process. The failure mechanism of this Super304H is the typical intergranular damage, the creep voids and micro-crack are mainly concentrated on the GBs and triple point GBs. Meanwhile, there is a lot of precipitates inside void chain, those particles are the M23C6 phase, which indicate that the coarsening and growth of M23C6 phase are detrimental to the microstructural property of Super304H. Besides, the GND density of Super304H increased remarkably at the creep acceleration period, and the dislocation structure also changed from the pattern of straight-line to the zigzag. In addition, the nonlinear ultrasonic measurement results showed that there is a general upward trend of nonlinear parameter β’ of Super304H during creep. The increase of the nonlinearity effect in Super304H could be due to several factors such as the precipitates precipitation, dislocation density/structure change, and the microdefects formation.

Oil–Water Two-Phase Flow Volume Fraction Measurement Based on Nonlinear Ultrasound Technique

Ultrasonic techniques have been widely used to determine the volume fractions of liquid components in oil–water two-phase flow. The two major ultrasonic interactions amenable to volume fraction estimation are the sound speed and attenuation. Instead of using these two linear ultrasound parameters, this study explores the use of ultrasound nonlinear parameter to measure the volume fraction of oil–water two-phase flow. Based on the theoretical modeling of ultrasound nonlinear propagation and nonlinearity measurement principle using finite amplitude insertion substitution (FAIS), we established a relationship between volume fraction and the amplitude of the second harmonic components in the received ultrasound waveform. With the use of high-intensity ultrasonic excitation, two-end calibration, and second harmonic amplitude extraction, the volume fraction of water can be calculated. The performance of the proposed method is validated by the numerical simulations and experimental studies in a lab-scale flow loop. Good qualitative agreement between the model and results shows that the nonlinear ultrasonic measurement technique has good quantitative accuracy and high measurement sensitivity, which make it well suited for industrial applications.

Numerical Study on Surface Roughness Measurement Based on Nonlinear Ultrasonics in Through-Transmission and Pulse-Echo Modes.

Ultrasonic is one of the well-known methods for surface roughness measurement, but small roughness will only lead to a subtle variation of transmission or reflection. To explore sensitive techniques for surfaces with small roughness, nonlinear ultrasonic measurement in through-transmission and pulse-echo modes was proposed and studied based on an effective unit-cell finite element (FE) model. Higher harmonic generation in solids was realized by applying the Murnaghan hyperelastic material model. This FE model was verified by comparing the absolute value of the nonlinearity parameter with the analytical solution. Then, random surfaces with different roughness values ranging from 0 μm to 200 μm were repeatedly generated and studied in the two modes. The through-transmission mode is very suitable to measure the surfaces with roughness as small as 3% of the wavelength. The pulse-echo mode is sensitive and effective to measure the surface roughness ranging from 0.78% to 5.47% of the wavelength. This study offers a potential nondestructive testing and monitoring method for the interfaces or inner surfaces of the in-service structures.

Investigation of the microstructure evolution in TP347HFG austenitic steel at 700°C and its characterization method

AbstractThis article reports the microstructure evolution in TP347HFG austenitic steel during the aging process. The experiments were carried out at 700°C with different aging time from 500 to 3,650 h. The metallographic results show that the coherent twin and incoherent twin are existed in the original TP347HFG grains, while they gradually vanished with the increase of the aging time. After aging for 500 h, a lot of fine, dispersed particles precipitated from the matrix, but they disappeared after aging for 1,500 h. When the aging time extend to 3,650 h, the precipitates appeared apparently coarse in TP347HFG steel, which include the M23C6andσphase; besides, the micro-hardness of TP347HFG also changes during the aging, which was closely related to the effect of dispersion strengthening and solution strengthening. The results of the nonlinear ultrasonic measurement reveal that theβ′ of TP347HFG steel was also changed with the aging time. It first increased at 0–500 h, then reduced later, and increased finally at 1,500–3,650 h. The variation ofβ′ in TP347HFG was influenced by a combined effect of the twin microstructure and the precipitate phase, which indicate that the nonlinear ultrasonic technique can be utilized to characterize the microstructure evolution in TP347HFG.

Nonlinear ultrasonic evaluation of high-density polyethylene natural gas pipe thermal butt fusion joint aging behavior

High-density polyethylene (HDPE) pipes are widely used in urban natural gas pipelines. Thermal butt fusion (BF) welds are the weakest components of pipelines. Welding joint aging causes degradation of the PE thermal BF joint material properties. Using the nonlinear ultrasonic technique as a nondestructive testing (NDT) method was proposed for evaluating aging damage. Nonlinear ultrasonic measurements were conducted for HDPE thermal BF joint samples exposed to compression creep. Ultrasonic harmonics are generated due to aging damage. The acoustic nonlinearity parameter β based on the fundamental and second harmonics is calculated. The results indicate that β is sensitive to the variation in HDPE specimens under different aging conditions. This NDT method is useful in tracking microstructural changes in BF joints due to aging during the service process.

Use of combined linear and nonlinear ultrasound to examine microstructural and microchemical variations in highly irradiated 304 stainless steel

This research combines linear and nonlinear ultrasound to examine the microstructural and microchemical changes in five large, highly irradiated cold-worked ANSI 304 stainless steel coin specimens cut from two hexagonal cross section blocks with radiation damage levels ranging from ~0.4 to ~33 dpa following irradiation in the EBR-II fast reactor. Both linear (velocity) and nonlinear ultrasonic (β) measurements were conducted in a hot cell using an automated fixture device specially designed to hold the sensor for repeatable measurements in an environment where direct human access is limited, demonstrating their potential to more fully investigate the complex defect ensemble introduced by irradiation. The measurements were performed at the Westinghouse Churchill site hot cell facility.The linear measurements agree very well with previous measurements employing a different sensor application technique. The ultrasonic nonlinearity parameters measured after irradiation increased by more than 100% from the unirradiated state and show a level of random variation below 4.7%. These ultrasonic nonlinearity parameters also show a spatial dependence on measurement location across each of the specimens in agreement with the spatial inhomogeneity of microstructure and microchemical distribution previously observed in adjacent, nominally identical specimens using transmission electron microscopy (TEM) and atom probe tomography (APT). It is proposed that the inhomogeneous distribution of microstructural and microchemical features including Frank loops, and both intragranular and grain boundary precipitates, as well as their different nonlinearity generation efficiencies are responsible for the spatial dependence of the measured ultrasonic nonlinearity parameters. The major and dominant component of the microstructure of these specimens at 20–30 dpa is vacancy agglomerations called voids, previously observed by microscopy and measurable using either linear ultrasonic velocity or linear attenuation, but which are not readily visible using nonlinear ultrasound. Components such as Frank loops, which are one of the other major microstructural components, do not contribute significantly to changing the linear ultrasonic velocity. Using this combination of linear and nonlinear measurements allows for the examination of microstructural/microchemical components whose linear ultrasonic interactions are overshadowed by voids, especially Frank loops and various radiation-induced precipitates.The changes in ultrasonic nonlinearity parameters in these specimens are thought to arise from two major sources. The smaller contribution is due to carbide pinning of dislocations in the grain boundary walls and the few line dislocations present in the grain interior. The largest contribution appears to come from segregation of Ni and Si to the perimeter of the very high density of Frank loops, leading to the formation of Ni3Si precipitates which are thought to be very efficient at pinning the Frank loops. These results on two hex blocks provide an ideal, though not completely realistic, test case to demonstrate the sensitivity of nonlinear ultrasound to two different microstructure sources.

Use of Higher-Harmonic and Intermodulation Generation of Ultrasonic Waves to Detecting Cracks due to Steel Corrosion in Reinforced Cement Mortar

The aim of this work was to provide further confirmation of the possible use of non-linear ultrasonic techniques for detecting the cracking due to corrosion of steel reinforcements in concrete. To this end accelerated steel corrosion tests have been conducted on model reinforced cement mortar specimens, while monitoring the appearance and width evolution of visible surface cracks, and performing non-linear ultrasonic measurements based on the phenomena of harmonic distortion and intermodulation. A new parameter, based on the difference between the amplitude of the fundamental frequency and the sum of the amplitudes of all the first-order and second-order intermodulation products, has been proposed in this work. The results confirm that the appearance of visible surface micro-cracks are preceded and accompanied by the observation of strong non-linear features in the received signal. Furthermore, the new parameter proposed in this work is as efficient as the relative non-linearity parameters, classically used in harmonic distortion non-linear ultrasonic studies, for detecting the non-linear features associated with the critical events of the cracking of cement mortar due to embedded steel corrosion. A hypothesis has been developed considering the possible effect of the filling of the void space by liquid containing rust products after the formation of new cracks or the enlargement of its width. This filling process, which might be particularly enhanced by net convective transport of liquid, would explain the evolution of the values of all the parameters used for putting in evidence the non-linear elastic features after the critical events of the cracking process.

Device and method for nonlinear ultrasonic measurements on highly irradiated 304 stainless steel specimens in a hot cell environment.

This paper reports on the first successful nonlinear ultrasonic measurement on highly irradiated specimens in a hot cell environment. The specimens are ANSI 304 stainless steel specimens for which the microstructure characterization and ultrasonic velocity measurement have been previously conducted. The critical part of this research is the development of an automatic fixture device that can facilitate repeatable loading and unloading to place the contact ultrasonic transducers on and off of the specimen. The key step to achieve high measurement repeatability is a careful adjustment of the support-spring constants such that the contact force at the interface between the transducer face and specimen surface is as uniform and constant as possible. The longitudinal ultrasonic velocities, which are obtained as a by-product of the nonlinear ultrasonic measurements, show a level of random variation in terms of (max-min)/average (%) below 0.2%, and the velocity distributions and magnitudes are in good agreement with those from the previous work. The ultrasonic nonlinearity parameters show the level of random variation below 4.7%, which is extremely low, considering that the measurements are conducted in a hot cell environment. The nonlinearity parameters also show a strong dependence on the measurement location in a particular specimen with respect to the radiation source, demonstrating a possible inhomogeneous microstructure evolution in these 12.7 mm thick specimens. This research demonstrates the feasibility of making nonlinear ultrasonic measurement on highly radioactive materials and/or in a highly radioactive environment using the device and procedure developed.

Comparison of Linear and Nonlinear Ultrasonic Parameters in Characterizing Grain Size and Mechanical Properties of 304L Stainless Steel

Ultrasonic nondestructive techniques can be used to characterize grain size and to evaluate mechanical properties of metals more practically than conventional destructive optical metallography and tensile tests. Typical ultrasonic parameters that can be correlated with material properties include ultrasonic velocity, ultrasonic attenuation coefficient, and nonlinear ultrasonic parameters. In this work, the abilities of these ultrasonic parameters to characterize the grain size and the mechanical properties of 304L stainless steel were evaluated and compared. Heat-treated specimens with different grain sizes were prepared and tested, where grain size ranged from approximately 40 to 300 μm. The measurements of ultrasonic velocity and ultrasonic attenuation coefficient were based on a pulse-echo mode, and the nonlinear ultrasonic parameter was measured based on a through-transmission mode. Grain size, elastic modulus, yield strength, and hardness were measured using conventional destructive methods, and their results were correlated with the results of ultrasonic measurements. The experimental results showed that all the measured ultrasonic parameters correlated well with the average grain size and the mechanical properties of the specimens. The nonlinear ultrasonic parameter provided better sensitivity than the ultrasonic velocity and the ultrasonic attenuation coefficient, which suggests that the nonlinear ultrasonic measurement would be more effective in characterizing grain size and mechanical properties than linear ultrasonic measurements.

A Novel Multi-frequency Nonlinear Ultrasonic Approach for the Characterization of Annealed Polycrystalline Microstructure

A multi-frequency nonlinear ultrasonic measurement is used to characterize grain size variations and distributions unambiguously. The ultrasonic nonlinearity parameter varies linearly with grain size in the Rayleigh scattering regime but deviates from linear behavior at the Rayleigh-to-stochastic transition zone. Frequency dependence of this parameter is found to be a reliable tool for rapid screening of materials where grain size varies widely.

Applicability of nonlinear ultrasonic technique to evaluation of thermally aged CF8M cast stainless steel

Abstract Cast austenitic stainless steel (CASS) is used for fabricating different components of the primary reactor coolant system of pressurized water reactors. However, the thermal embrittlement of CASS resulting from long-term operation causes structural safety problems. Ultrasonic testing for flaw detection has been used to assess the thermal embrittlement of CASS; however, the high scattering and attenuation of the ultrasonic wave propagating through CASS make it difficult to accurately quantify the flaw size. In this paper, we present a different approach for evaluating the thermal embrittlement of CASS by assessing changes in the material properties of CASS using a nonlinear ultrasonic technique, which is a potential nondestructive method. For the evaluation, we prepared CF8M specimens that were thermally aged under four different heating conditions. Nonlinear ultrasonic measurements were performed using a contact piezoelectric method to obtain the relative ultrasonic nonlinearity parameter, and a mini-sized tensile test was performed to investigate the correlation of the parameter with material properties. Experimental results showed that the ultrasonic nonlinearity parameter had a correlation with tensile properties such as the tensile strength and elongation. Consequently, we could confirm the applicability of the nonlinear ultrasonic technique to the evaluation of the thermal embrittlement of CASS.